Apparatus for eliminating effects of electrostatic discharge

ABSTRACT

Apparatus is described for electrostatically discharging personnel prior to their contact with sensitive electronic equipment. All parts of the equipment they come in contact with are first electrically connected with a ground network. To reduce the discharge current to a non-parasitic magnitude a currentlimiting discharge resistance is connected intermediate the aforementioned and the ground network. However, this section can, likewise, be composed of a material having an appropriate conductance.

United States Patent 11 1 Menzel et a]. 1 51 Nov. 4, 1975 1 APPARATUS FOR ELIMINATING EFFECTS 3.594.490 711971 Mitchell et d1 174/51 0 ELECTROSTATIC DISCHARGE 3.711.742 1/1973 Pinkharn 317/2 R 3.780.345 12/1973 Earmam 317/7. R [75] Inventors: Willi Menzel, Gauting; Peter Bauer,

Munich. both of Germany [73] Assignee: Siemens Aktiengesellschaft, Berlin & DH-{7W3} 'f Mlucr Munich Germany ASSLYIUHI EmmumrHzirry E. Moose, Jr.

[22] Filed: Mar. 13, 1974 l 1 2. Appl. No. 450,740 [57] ABSmCT [30] Foreign Application Priority Data Apparatus is described for electrostatically discharg- Mar. 13, 1973 Germany 2312469 ing personnel prior to their contact with sensitive electronic equipment. All parts of the equipment they [52] U.S. Cl 317/2 R; 174/5 R; 174/51 come in contact with are first electrically connected [51] Int. Cl. HOSF 3/02 with a ground networkv To reduce the discharge cur- [58] Field 0fSearch...3 17/2 R,2 B, 10] CB, 10] DH: rent to a n0n-parasitic magnitude a current-limiting 174/5, 6, 51; 150/52 R, 52 K, 52 L; 220/85 discharge resistance is connected intermediate the 16/116 R aforementioned and the ground network. However, this section can, likewise, be composed of a material :56] References Cited having an appropriate conductance.

UNITED STATES PATENTS 3.099.774 7/1963 Crane 317/2 R 2 Claims, 2 Drawing Figures US. Patent Nov. 4, 1975 3,917,978

APPARATUS FOR ELIMINATING EFFECTS OF ELECTROSTATIC DISCHARGE BACKGROUND OF THE INVENTION The rooms and other areas for electrical equipment, such as communication facilities, computers, and the like containing digital sequential circuits, electric storages, integrated circuits and similar highly sensitive electronic components are increasingly being provided with highly insulating plastic floor coverings, e.g. vinyl floor tile. These coverings lead to considerable static charges on persons through frictional electricity, particularly if the hygroscopic moisture in the rooms is maintained especially low through air conditioning systems. However, due to the very high charging voltage (l to kV), people charged with static electricity store electric energies sufficient to destroy, for example, integrated circuits upon direct contact. Such discharge surges may, likewise, actuate digital sequential circuits, e.g. erase or set storages, if the discharge current passes in the vicinity of such a sequential circuit. This leads inevitably to error functions in the equipment.

In addition, sensitive people perceive a disagreeable, though harmless, shock when the discharge takes place.

To prevent an electrostatic charge, it is generally recommended that adequate air humidity be provided in the space in question. Antistatic spray or, under certain circumstances, treatment with cleansing foam for carpets can provide temporary relief. Such steps are appropriate for apartments and the like, but not for spaces in which electrical equipment of the type referenced hereinabove is located. Excessive air humidity and foreign substances would contaminate the pieces of equipment (e.g. contacts) and would also lead to undesirable leakage currents.

It is known in the art to provide modules in communication facilities with conductive handles or the like which are electrically connected to the ground network of the equipment. This insures the discharge of maintenance personnel upon contact with the shielding, thereby protecting the sensitive component parts they subsequently come in contact with adequately against destruction. However, this measure cannot prevent the effect of the electric shock on the personnel. Moreover, there briefly arise high currents, upon the discharge which during the current flow through the ground network of the equipment causes considerable dynamic voltage drops. These voltage differentials in the ground network of a piece of equipment unavoidably disturb the operations in digital sequential circuits, which can lead to a brief breakdown of the equipment.

It is, therefore, an object of the invention to provide a means for eliminating the effect of electrostatic discharge of persons on themselves, as well as on the highly sensitive electronic components.

SUMMARY OF THE INVENTION The invention has application, inter alia, with electrical equipment equipped with highly sensitive electronic parts or modules, such as communication facilities, computers, etc. containing digital sequential circuits, electric storages, integrated circuits, etc., having parts connected to a ground network, over which the mainte- 2 nance personnel charged with static electricity are discharged.

The invention solves the problem described herein above by inserting a resistance between the parts of the equipment and the ground network. The resistance is of a value which permits discharge currents of the static electricity which at the very most are as large as the changes in the operating current occurring in the equipment.

By inserting a resistance between the pieces of equipment which are seized first and the ground network, the static electricity of the maintenance personnel is discharged as in the case of direct grounding. However, the discharge time is increased so that no shock is per ceived.

A value for the discharge currents is obtained, by properly determining the resistance, which, at the very most, is as large as the changes in the operating current occurring in the equipment. This value is no longer adequate to destroy the built-in electronic components or to initiate any switching operations.

Let it be assumed, as a example, that a person were charged to 10 kilovolts. The human body capacitance lies at about 300 picofarads. With a resistance value of I00 kilohms the maximum discharge is milliamperes. In this case, the discharge process is terminated in about 100 microseconds. Such a current cannot affect an equipment operation so that further known steps, such as division of the ground network into a system earth and a safety earth, are necessary.

According to an advantageous further development of the invention it is also possible to make the pieces of equipment in question, e.g. the handles thereof, of a material whose resistance value corresponds to the re sistance described hereinabove. This resistance is inserted between the parts and the ground network. Such a handle could, for example, be made of plastic with a low conductance. This form of construction has the ad vantage that one need not pay attention to an insulated attachment of the highly conductive shield to earthed equipment and a secure fastening of the resistor.

BRIEF DESCRIPTION OF THE DRAWINGS The principles of the invention will be best under stood by reference to a description of preferred embodiments, constructed according to those principles, given hereinbelow in conjunction with the drawings in which:

FIG. 1 is a partial side elevation ofa handle and cabinet arrangement on a plug-in unit and FIG. 2 is a partial side elevation of an alternate preferred embodiment employing direct connection.

DETAILED DESCRIPTION OF THE DRAWINGS As shown in FIG. 1, a metal handle I is attached in a known manner, by screws 6, to a plug-in unit 4 of metal together with a metal shield 2. An insulating intermediate member 3 is inserted for electrically separating the handle 1 and the shield 2 from the plug-in unit 4, insulating discs being mounted underneath screws 6.

The outer metal parts 1 and 2 are connected to a printed conductor path 8 via a resistor 5 to point 7 which, in turn, is grounded within the equipment. The electrostatic voltage applied to the handle 1 or to the shield 2 can be discharged to earth via the resistor S and the printed conductor 8.

In order not to employ the discrete resistor 5 and the insulated attachment of the handle and of the shield,

the handle 11 and the shield 12 are connected directly and electrically to the plug-in unit 14, as shown in FIG. 2. In this case. handle 11 and shield 12 consist ofa ma terial having a specified conductivity, and thus, reduce the discharge current upon contact with the handle 11 or shield 12 to a value such as, likewise, is effected by the resistor 5 shown in FIG. 1.

It is irrelevant whether the handle 11 and the shield 12 are made of one piece or are put together separately, and whether the shield 12 is connected to the grounded plug-in unit 14. What is of importance is that a current conducting connection is created which can be achieved without difficulty by one skilled in the art. In either case, one has to pay attention to the fact that the printed conductor 8 or the plug-in unit 14 is properly connected to the ground network, even if the plugin units are partially pulled out.

The plug-in unit 14, as shown in FIG. 2, may also have above and/or underneath an additional frame made of the same material as the shield 12 and which is connected thereto or directly to the plug-in unit 14. In this way, the highly sensitive electronic components, of which only a few are shown in transistor cans 9, are better protected against random contact when the plug-in unit is pulled out. Also the plug-in unit 4 shown in FIG. 1 can be supplemented in the say way.

The preferred embodiments described hereinabove are intended only to be exemplary of the principles of the invention and are not to be considered as being determinative of the scope of the invention. Reference must be had to the appended claims for the latter purpose. Further, it is contemplated that modifications or 4 changes in the described embodiments can be made while remaining within the spirit and scope of the invention, as defined by the claims.

We claim:

1. ln electrical equipment having components which will respond to static electricity from an external source and having a discharge path from structural members which might be contacted by an operator or the like receive said static electricity from said external source, apparatus for controlling, at all times and without requirement for further action, the discharge currents flowing through said discharge path, comprising:

resistance means having a value which will permit the flow of discharge currents having a value up to the changes in the operating current occurring in said equipment, said resistance means being connected between the said structural members which might be contacted by an operator and said discharge path.

2. Electrical equipment having components which will respond to static electricity from an external source, comprising:

structural members for receiving said static electricity from said external source and a discharge path which is conductive at all times from said structural members to ground network,

said structural members being composed of a material having a resistance value which will permit the flow of currents to said discharge path which, at a maximum, as large as the change in the operating current in said equipment.

* l l l I 

1. In electrical equipment having components which will respond to static electricity from an external source and having a discharge path from structural members which might be contacted by an operator or the like receive said static electricity from said external source, apparatus for controlling, at all times and without requirement for further action, the discharge currents flowing through said discharge path, comprising: resistance means having a value which will permit the flow of discharge currents having a value up to the changes in the operating current occurring in said equipment, said resistance means being connected between the said structural members which might be contacted by an operator and said discharge path.
 2. Electrical equipment having components which will respond to static electricity from an external source, comprising: structural members for receiving said static electricity from said external source and a discharge path which is conductive at all times from said structural members to ground network, said structural members being composed of a material having a resistance value which will permit the flow of currents to said discharge path which, at a maximum, as large as the change in the operating current in said equipment. 